Hi I was wonder what the limitations would be (if any) if I were to crossfire an unlocked 6950 to a 6970 spec, and a 6950. I know I have been told that the more powerful gpu will 'downgrade' to the lower gpu spec. But I'm not sure what the details in that are. Can someone shed some light on this? Thanks.

It will work together just fine. However the performance will be somewhere between 6950 and 6970 crossfire.
It may be advantagous to just run them both as 6950's to elimanate load imbalances. I tried a 6870 with a 6850 once and while it worked fine, there was some definite rubberbanding in the frame rates. Try it both ways and use it whichever way works best.